Traditionally, additives such as anti-foulants, anti-scaling agents, corrosion inhibitors, buffering and pH agents, microbiocides and the like are added directly to the solutions of aqueous systems as needed to prevent scale deposition, corrosion of metal surfaces and similar fouling of the aqueous systems, as well to maintain proper pH levels. As used herein, an aqueous system may include, without limitation, a cooling system, an open circulating cooling water system and an engine cooling system. Also, as used herein, a “system” includes at least a circulating solution. For example, a system may include a circulating solution, pump, tubing, etc.
In certain aqueous systems, it is important to maintain a steady level of additives. For example, the presence of microbiocides is especially important in an aqueous system such as cooling systems employed in cooling towers. Cooling towers usually maintain a cooling system for a considerable length of time. Typically, such cooling systems do not have sufficient aeration and exposure to sunlight to prevent microbial, especially bacterial and fungal, growth. In particular, many cooling systems use fill composed of synthetic polymer or other materials, for example, corrugated polyvinyl chloride and the like materials, in order to extend the amount of heat exchange surface area.
This type of construction greatly aggravates the problem of microbiological growth, since it provides an ideal physical environment for the propagation of troublesome microbes. If left untreated, such microorganisms may flourish and produce colonies extensive enough to give rise to problems of biofilm blockage of heat exchange surfaces, as well as clogging of the components of the water transporting apparatus used in operating the aqueous system.
Various methods of introducing additive to an aqueous system have been developed. For instance, a solid additive material may be added directly to the aqueous system which dissolves in the aqueous system. However, this method cannot maintain a steady concentration level of additive within the system. Initially, there would be a high level of the additives released into the system, and within a short time the additives are depleted. Additionally, a significant draw back of this method is the danger of overdosing the system with particular additives which are initially released. The overdosing is dangerous in that it can result in erosion and corrosion problems.
Various attempts have been made in the prior art to address particular water treatment systems by using controlled release coatings. For example, Characklis in U.S. Pat. No. 4,561,981 (issued Dec. 31, 1985) disclosed a method for controlling, preventing or removing fouling deposits, particularly in pipelines, storage tanks and the like by microencapsulating fouling control chemicals in a slow release coating. The coating material is described as being any material compatible with the fouling control chemical which is capable of sticking to the fouling deposit site. However, the coating materials as disclosed by Characklis may dissolve in a cooling system and create further corrosion problems.
Mitchell et al. in U.S. Pat. No. 6,010,639 disclosed using various polymers as coatings for coolant additives. Also, Blakemore et al. in U.S. Pat. No. 6,878,309 disclosed that copolymers derived from two different ethylenically unsaturated monomers may be used as coatings for coolant additives.
There continues to be a need for new controlled release cooling treatment compositions.